Young's double slit gone wrong

  1. Hi Everyone

    I was trying to do a young's double slit experiment with green laser.

    The gap between the slits was d = 0.001m

    The distance to the screen was D =1.2m

    We counted from the center 10 bright fringes m = 10

    The distance of the from the center bright fringe was y = 0.067 m

    Using a small angle approximation: y = (mλD)/d

    So λ = (yd)/mD = (0.067 x 0.001)/12 = 5.5 x 10^-6 Wavelength of laser given as about 550 nm = 5.7 x 10^-7

    ouch! out by a factor of ten. Can't figure out what I did wrong.

    NB: This is not a homework question. I am just trying to make the experiment work, and though I am ashamed of my incompetence, I am actually the teacher!

    Thanks in advance for any suggestions. The bright spot in the middle was a bit big and smudged. But that was partly why we chose to count as many as 10 fringes. Even if for some reason there were an extra one or two fringes that got burnt out at the center, that would not account the factor of ten.
     
  2. jcsd
  3. UltrafastPED

    UltrafastPED 1,919
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    Was this done on an optical table? Do you know the coherence length of your laser?

    You will get the best results with a HeNe on a table which cannot vibrate ... the basement floor is better than a lab bench on the second story!

    From your use of a green laser I suspect you are using a laser pointer ... in that case you may also have a problem with the fixturing.

    I did not check the math ... I was immediately drawn to the smudged central spot.
     
    Last edited: Sep 7, 2013
  4. Thanks. The thing was/is that the pattern of bright and dark fringes was very clear. The set up was crude. But I don't understand why I should be so neatly out by a factor of ten. I think I would like to try some more simple single slit diffraction experiments and set what happens there. I wasn't really looking for ultra precision, just a quick demonstration to show the students and hopefully to get some practice with the calculations. In fact I am beginning to wonder whether what I was actually looking at was the diffraction pattern cause by only one slit. The slits themselves were about 1/10 th of a millimeter. Thanks, I will go back and try again.
     
  5. UltrafastPED

    UltrafastPED 1,919
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  6. jtbell

    Staff: Mentor

    Yes, I suspect you're counting the single-slit maxima, not the double-slit maxima. If the width of the slits is about 1/10 the spacing between them, that would account for the numbers you're getting. Try increasing D to a few meters and doing the experiment in a dark room. That spreads out the pattern and you can distinguish more easily between the single-slit and double-slit components of the interference pattern:

    http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/dslit.html
     
  7. sophiecentaur

    sophiecentaur 13,389
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    That seems likely to me, too. The width / spacing ratio was 1:10. Coherence shouldn't be a prob as you can do Young's Slits with a discharge lamp and a single collimating slit. Just increase the throw and the finer fringes should jump out at you.
     
  8. Redbelly98

    Redbelly98 12,029
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    If this is a single-slit diffraction pattern, then the spacing between the two dark fringes nearest the center will be twice the spacing of other adjacent fringes. For a double slit, the spacings would be equal.
     
  9. StevieTNZ

    StevieTNZ 934
    Gold Member

    Done in open air, decoherence?
     
  10. I did the experiment again. This time I drilled a single hole through a piece of metal. The whole was 0.3 mm and the resulting interference pattern was easy to see. The calculation produced a result a result that was about 95% accurate. This was done with a green laser pointer clamped in a regular lab clamp stand, and projected over a distance of just over a meter. So problem solved. Thanks everyone for your replies and advise
     
  11. Redbelly98

    Redbelly98 12,029
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    That is one small drill bit. Just FYI, the pattern from a single hole is considered a diffraction pattern, not an interference pattern.

    Glad it worked out.
     
  12. sophiecentaur

    sophiecentaur 13,389
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    The light doesn't care what you call it. Is there any practical example of 'pure' interference?
    People are obsessed with classification.
     
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